1. Field of the Invention
This invention relates to a focal plane shutter which permits highly precise adjustment of the degree of parallelism between an end part of the front screen of the shutter and an end part of the rear screen of the shutter which form a slit between them for exposure.
2. Description of the Prior Art
As is well known, a focal plane shutter normally operates to form a slit between two screens which are made of rubber coated fabric or a metal and which are disposed immediately in front of a photographing film for exposing it to light.
Since the focal plane shutter is arranged to have the shutter screens travel at an approximately constant speed, the travelling speed of the slit formed by them is also fixed. Accordingly, the degree of film exposure to light is determined by electrically or mechanically varying the width of the slit to make it longer or shorter. However, since the width of the slit is determined by the end part of one shutter screen and that of the other shutter screen, one end of the slit and the other end of the slit will differ in width if the end part of one shutter screen is not parallel with that of the other shutter screen. In such a case, the upper side of a picture thus exposed on the film would have a different degree of exposure from the lower side thereof. To avoid such uneven exposure, therefore, the end part of one shutter screen must be adjusted to be in parallel with that of the other shutter screen.
Therefore, the parallelism between the end parts of the two shutter screens forming a slit has heretofore been adjusted, as shown in FIG. 1, when rollers 2 and 3 which are fixed to ribbons 1a and 1b are tightly secured to a front screen shaft 4 by means of screws 5a and 5b. Referring further to FIG. 1, a slit A is formed between the slit forming end part 1.sub.1 of a front shutter screen 1 and the slit forming end part 6.sub.1 of a rear shutter screen 6. However, if the slit forming end part 1.sub.1 should fail to be in parallel with the other slit forming end part 6.sub.1, the width of the slit A in the upper part thereof would differ from the width in its lower part. Therefore, in the final process of assembly of a shutter, these shutter screens have heretofore been adjusted to have their slit forming ends in parallel with each other by first attaching one of the rollers 2 and 3 and then attaching the other of the rollers 2 and 3 with the screws while effecting adjustment as required. However, in accordance with this method for adjusting the parallelism between the slit forming ends of the two shutter screens by tightening the screws 5a and 5b to fix the rollers 2 and 3 to the front shutter screen shaft 4, once the rollers 2 and 3 are tightly fixed to the front screen shaft by means of the screws 5a and 5b, tightening traces are left on the front shutter screen shaft. After that, in carrying out subsequent fine adjustment, when the rollers are slightly turned by loosening the screws 5a and 5b and then by again tightening the screws 5a and 5b, the traces produced by previous tightening impedes subsequent tightening of the screws 5a and 5b. Thus, it has been difficult to make fine adjustment in accordance with the above mentioned method. In view of this, in another method, instead of using the screws as shown in FIG. 1, the ribbons of the front screen are attached to the rollers with the slit forming ends of the shutter screens adjusted to be in parallel with each other to ensure that the slit has equal widths at its upper and lower ends. This adjustment is done before the adhesives for adhering the ribbons on the rollers are solidified. In the latter method for adjusting the parallelism, however, a spring force of the spring drum of the front shutter screen is exerted after the ribbons of the front screen are attached to the rollers to cause the connections between the ribbons and the rollers to deviate from their set positions. Therefore, it has also been difficult to make precise adjustment in accordance with the latter method.